Lost foam casting (also called evaporative pattern casting and expendable pattern casting) evolved from the full mold process following the general availability of expanded polystyrene foam. In full mold casting, a bonded sand mold is formed around a foam pattern cut to the size and shape of the desired casting. Liquid metal is poured directly into the pattern, causing the foam to melt and then vaporize under the heat of the metal. Air and polymer vapor escape from the mold cavity through narrow vents molded into the sand above the pattern, allowing the liquid metal to displace the entire volume originally occupied by the foam. The full mold process is particularly useful for making large, one-off castings such as metal stamping dies.
The main difference between lost foam casting and the full mold process is that in lost foam casting the mold is made from loose sand, which is consolidated around the pattern by vibration. Vents are not required because the foam decomposition products are able to escape through the natural interstices between the sand grains. Patterns are molded to shape rather than cut from a larger foam block, and sometimes they are glued together from two or more pieces when internal passages do not allow them to be molded as one. After the pattern is assembled, it is dipped in a water-based refractory slurry and allowed to dry. This forms a porous coating on the surface of the pattern, which keeps the metal from penetrating the sand while still allowing the foam decomposition products to escape from the mold cavity. The coated pattern is then placed inside a steel flask and surrounded with loose, dry sand. Next, the flask is vibrated to consolidate the sand and encourage it to fill any open passages in the pattern. After that, liquid metal is poured into the pattern, which gradually gives way to the hot metal as its gas and liquid decomposition products diffuse through the coating and into the sand. Once the casting solidifies, the sand is poured out of the flask and the casting is quenched in water.
In the past few years, some lost foam foundries have begun using synthetic ceramic media in place of silica sand primarily because of its superior durability and its more insulative thermal properties. Here, the term sand is used in a generic sense to refer to any type of granular mold media.
As a process for making complex parts in high volume, lost foam casting has several important advantages. First, the molds for the foam patterns are relatively inexpensive and easy to make. Castings are free from parting lines, and draft angles can be reduced or even eliminated. Internal passages may be cast without cores, and many design features, such as pump housings and oil holes, can be cast directly into the part. Lost foam casting is more environmentally sound than traditional green sand casting because the sand can be cleaned and reused.
Unlike traditional casting processes (such as lost wax casting) where metal is poured directly into an empty mold cavity, the mold filling process in lost foam casting is controlled more by the mechanics of pattern decomposition than by the dynamics of metal flow. The metal advances through the pattern only as fast as foam decomposes ahead of it and the products of that decomposition are able to move out of the way. Before any liquid metal can flow into the cavity, it must decompose the foam pattern immediately ahead of it. As it does, some of the foam decomposition products can mix with the metal stream and create anomalies such as folds, blisters, and porosity in the final casting.
Lost foam casting has been used successfully with aluminum, iron, bronze, and more recently magnesium alloys. In the auto industry, for example, aluminum is used to make engine blocks and heads. Currently, more experimental data is available for aluminum than for any other material.
In spite of its many advantages, lost foam casting is still prone to fill-related process anomalies due to foam decomposition products that are unable to escape from the mold cavity before the casting solidifies. These anomalies are divided into four main categories. Gas porosity is created when foam decomposition products remain trapped inside the metal as it solidifies. Blisters form on the upper surfaces of castings when rising bubbles are trapped below a thin surface layer of solidified metal. Wrinkles form on casting surfaces when residual polymer liquid is caught between the metal and the coating and cannot escape before the casting solidifies. Sometimes, though, even when all the foam decomposition products do escape from the mold cavity, they still leave folds in the casting. A fold is a pair of unfused metal surfaces, usually contaminated by oxides and carbon residue, left behind when a pocket of polymer liquid or gas collapses on itself.